Measurement of Conductivity and pH inPhotovoltaic HF Etching Solutions

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Hydrofluoric acid (HF) is used in photovoltaic processing, glass etching and other industrial pro-cesses. The extremely corrosive and toxic natureof HF makes it difficult to handle, requiring special materials of construction. This is also true of the sensors needed for its measurement. The strongetching baths as well as the more dilute rinse waters and wastewater neutralization processes require careful selection of sensors for conductivityand pH.

Etch BathsHydrofluoric acid is commercially available at concen-trations near 50%. In etching solutions this concen-tration must be diluted and maintained within close tolerances to achieve controlled etching rates and high yields. This concentration can be monitored using inductive conductivity instrumentation which allowsmeasurement without any metal in contact with the fluid. The sensor consists of two adjacent coils encap-sulated in a single polymer body. The first coil is ener-gized with AC current. The fluid surrounding the dough-nut shaped sensor induces a signal into the second coil in proportion to the conductivity of the fluid. This signal is then measured, temperature compensated and provided for display and control. Inductive con-ductivity sensors using PFA (perfluoro-alkoxy) and PTFE (polytetrafluoroethylene) wetted materials with-stand the severe conditions of HF-containing baths.

Rinse WaterPV cells and other products etched in hydrofluoric acid baths must be rinsed extensively before further proces-sing. Usually this is done with ultrapure water. The initial rinse water will contain enough HF to be highly corrosive while the final rinse must reach ultrapure water levels of purity. To assure thorough rinsing, an ultrapure level contact conductivity/resistivity sensor must monitor the used water quality. At the same time, this sensor must withstand the initial acid conditions.

Resistivity sensors for monitoring etching rinse water are available with Monel® electrodes which have excellent resistance to HF, whereas standard stainless steel or titanium sensors are rapidly attacked. Monel sensors are calibrated and measure with the same high accuracy available with other Thornton ultrapure resistivity sensors and will tolerate occasional dumps of hydrofluoric acidetchant as well as the dilute rinse water. UniCond™ con-ductivity sensors measure over a wide range, allowing extended monitoring of HF rinsing processes.

Fig 3.

Wastewater TreatmentThe rinse waters and the etch baths themselves, when they are spent, must be treated before discharge or reclaim. This treatment typically consists of neutraliza-tion, coagulation and settling of the fluoride by adding calcium chloride, lime and/or limestone to produce insoluble calcium fluoride. The measurement of pH to control the neutralization is complicated by the corro-sive nature of acidic fluoride solutions. HF attacks the glass membrane of most pH sensors as readily as it etches photovoltaic cells. The rate of attack is depen-dent on fluoride concentration, pH and temperature as well as the glass composition. If the fluorides areconsistently neutralized upstream of the pH sensor,then there is less opportunity for attack.

The exposure to acidic conditions may depend on the treatment strategy. If treatment is done in batches as in Figure. 1A, with filling, neutralizing and pumping out, then the pH sensor could be exposed to hydrofluoric acid all through the long filling cycle. In those situa-tions a special HF-resistant glass pH sensor may be needed to achieve greater resistance to hydrofluoric acid. On the other hand, if treatment is continuous with gravity overflow as in Figure 1B, then control couldmaintain the pH near neutral most of the time and the pH sensor would not be subjected to hydrofluoric acid. A conventional glass electrode should be satisfactory in this case unless there is poor mixing or frequent control upsets occur.

Fluoride treatment typically involves precipitation of calcium fluoride which can rapidly coat any kind of pH sensor as well as the process equipment itself. The pH sensor installation should be designed to enable easy access to the electrode for maintenance which usually includes cleaning with dilute hydrochloric acid (HCl) to dissolve the calcium scaling.

The pH vs. fluoride concentration in Figure. 3 shows the initial recommendation for sensor type for various operating conditions at 20°C (68°F). It is always desirable to use a conventional glass pH sensor which provides accurate measurement at moderate cost. The alternative is the HF-resistant glass sensor (Fig 2) which has a glass formulation more resistant to HF but is higher in cost. This figure is only a guide and cannot account for variations in temperature, additional materials present or other conditions which may affect the rate of attack of a glass sensor. Other factors may also be important in selecting the best sensor for a particular application, but this provides a starting point.

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Fig 2. HF-resistant pH electrode formost HF neutralization processes

Fig 1A. Batch Fig 1B. Continuous

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Fig 2. HF-resistant pH electrode formost HF neutralization processes

HF-Resistant SensorsInductive Conductivity for HF Etchant Concentration52 005 424 7250 PFA Inductive Conductivity Sensor with 5 m (16 ft) cable52 005 425 7250 PFA Inductive Conductivity Sensor with 10 m (33 ft) cable

Contact Conductivity with 0.1 cm-1 cell constant for HF Rinse Water230-251 Monel conductivity sensor with 34 mm (1.35”) insertion used with 770MAX Transmitter230-261 Monel conductivity sensor with 132 mm (5.19”) insertion used with 770MAX Transmitter240-203 Monel conductivity sensor with 34 mm (1.35”) insertion used with M300 Transmitter240-204 Monel conductivity sensor with 132 mm (5.19”) insertion used with M300 Transmitter58 031 407 ISM Monel conductivity sensor with 34mm (1.35”) insertion used with M800 andM300 ISM Transmitters58 031 408 ISM Monel conductivity sensor with 132mm (5.19”) insertion used with M800 andM300 ISM Transmitters

Glass pH Electrode for HF Neutralization52 003 550 pH Electrode, 4262/120/Pt1000 VP HF Resistant, may be used with THORNTONhousings and instruments including the M300 and 770MAX with preamplifier.

Monel® is a registered trademark of American Special Metals Corp.

UniCond™ is a trademark of METTLER TOLEDO

www.mt.com/thornton

Mettler-Toledo Thornton, Inc.36 Middlesex TurnpikeBedford, MA 01730 USATel. +1-781-301-8600Fax +1-781-301-8701Toll Free +1-800-510-PURE (US and Canada only)thornton.info@mt.comAN0126 3/2011